The present invention relates to improved polymer compositions which are particularly suitable for use as pressure sensitive adhesives. By the use of a combination of hydroxyalkyl (meth)acrylate and itaconic acid monomers in polymers containing primarily monovinylidene aromatic and conjugated diene monomers, surprising combinations of adhesion and shear strength are obtained.
As used in the art, the term "pressure sensitive adhesives" designates a polymeric material which, when dried, is aggressively and permanently tacky at room temperature. A typical pressure sensitive adhesive will firmly adhere to a variety of dissimilar surfaces upon mere contact without the need of more than finger- or hand-applied pressure. The most important properties recognized by the pressure sensitive adhesive industry are the adhesion ("tack") and resistance to shear of these polymer compositions. Attempts to improve the tack properties of a pressure sensitive adhesive, such as by the addition of a tackifier, unfortunately tend to reduce the resistance to shear and promote shear failure.
Recent efforts to optimize pressure sensitive adhesive properties have focused on the use of acrylate-type latex polymers and the use of minor amounts of hydrophylic monomers therein. In EP 48950 latex polymers based on acrylate and diene monomers are modified with minor amounts of an unsaturated acid to improve their pressure sensitive adhesive properties. In Japanese Patent 59-47,212 similar acrylate-diene polymers are taught which attempt to replace the major part of the acid with a hydroxyalkyl acrylate monomer. Japanese Patent publication 61-141,775 (1986) discloses the use of specific oligomeric emulsifier compounds with pressure sensitive adhesive latex polymers based on styrene and butadiene and further containing optionally methyl methacrylate, itaconic acid and hydroxyethyl acrylate. However, it has been found that (meth)acrylate-containing polymers can possess an undesirable odor and there is always an interest in finding adhesive polymers with improved physical property combinations.
Efforts to improve the properties of monovinylidene aromatic-conjugated diene adhesive polymers have involved the use of acid, (meth)acrylate and/or (meth)acrylonitrile monomers. See for example U.S. Pat. Nos. 4,540,739 and 4,445,959 and E.P. 22 967. In efforts to prepare pressure sensitive adhesives from these non-acrylate polymers, however, it has been found to be difficult to obtain good combinations of tack and shear resistance.